Electroluminescent displays (ELDs) have already been used in emissive electronic display devices. One of components of an ELD is an inorganic electroluminescent element or an organic electroluminescent element (hereinafter, referred to as an organic EL element). The inorganic electroluminescent element has been used as a planar light source. Such an emissive element, however, requires high voltage of alternating driving current.
The organic EL element has a structure of an emissive layer containing an emissive compound interposed between a cathode and an anode. Electrons and holes are injected, into the emissive layer and are recombined to form excitons. When, this exciton is deactivated, emission of light (fluorescence or phosphorescence) occurs. This organic EL element can emit light by such a phenomenon at a voltage of several to several tens of volts. Moreover, the organic EL element has a wide viewing angle and a high visibility because it is a self-luminescent element. Furthermore, the organic EL element, which is a thin-film type of full solid-state element, is attracting attention from the points of view of space saving and of portability.
An example development of the organic EL elements for practical use is an organic EL element utilizing phosphorescent emission from an excited triplet state reported by Princeton University (M. A. Baldo et al., nature, Vol. 395, pp. 151-154, 1998). As described in U.S. Pat. No. 6,097,147 and M. A. Baldo et al., nature, Vol. 403, No. 17, pp. 750-753 (2000), research on materials to generate phosphorescence at room temperature has been carried out more actively since then.
Moreover, a recently discovered organic EL element utilizing phosphorescent emission can achieve emission efficiency, in principle, about four times higher than those of previous elements utilizing fluorescent emission. Research and development works on the layer structure of emissive elements and electrodes are being carried out worldwide as well as development work on materials for the recently discovered organic EL element.
For example, many compounds have been synthesized and examined mainly for heavy metal complexes such as a series of iridium complexes, and have been used for an emissive layer in an organic electroluminescent element (also called as an organic EL element) as described in S. Lamansky et al., J. Am. Chem. Soc., Vol. 123, p. 4304 (2001), for example.
Although an organic EL device utilizing phosphorescent emission is a system of great potential, major technical issues for the device are the way of controlling the position of the emissive center, particularly stable recombination inside the emissive layer and stable emission of light, as well as enhancement of the emissive property of a phosphorescent compound itself, from the viewpoints of efficiency and lifetime of the element.
In order to enhance the emissive property of a phosphorescent compound, there are two possible approaches: (1) increasing the radiative rate constant (kr) and (2) decreasing the non-radiative rate constant (knr), when the lowest excited triplet state (T1) is deactivated to the ground state (S0).
A possible specific means for decreasing the non-radiative rate constant (knr) is to sterically control the structure of a ligand of the phosphorescent compound to decrease structural changes between the ground state and the excited state.
With regard, to the iridium complex, which is a typical phosphorescent compound, examples in which the steric structure is controlled by a combinated ligand of dibenzofuran and pyridine are described in, for example, Japanese Patent Application Laid-Open Publication Nos. 2002-332291, 2005-23071 and 2002-23072.
Similar applications are described for iridium complexes formed with phenylpyrazole derivatives (see Patent Literatures 1 and 5), phenylimidazole derivatives (see Patent Literatures 2 and 3), and derivatives containing a carbene moiety as a ligand (see Patent Literature 4 and Non Patent Literature 1).
A platinum complex including a ligand having π (pai)-conjugation extending over the benzene ring of the ligand is synthesized (Non Patent Literature 2).
These complexes are, however, not satisfactory in terms of providing an organic EL element that has nigh emission efficiency and low drive voltage, excels in heat endurance and raw storability, and has a long lifetime. A further solution is therefore being sought.